Known carriers include those coated with a copolymer comprising vinylidene fluoride (hereinafter generally referred to as "VdF") and tetrafluoroethylene (hereinafter generally referred to as "TFE") (Japanese Unexamined Patent Publication No.58-20,875).
The carrier coated with the VdF/TFE copolymer has excellent properties such as larger electrostatic charge capacity than the silicone-coated carrier and higher electrostatic charge stability due to lower adhesion of the toner to the carrier. However, when the carrier is used in combination with a toner which aims to give improved properties indicated as above to the silicone-coated carrier, the resultant electrostatic image developer exhibits a serious defect that images cannot be transferred because of too large an electrostatic charge capacity. When a toner capable of being used both for the silicone-coated carrier and the VdF/TFE copolymer coated carrier is to be developed without imparing the excellent properties of the carrier as stated above, the toner would be very costly and economically disadvantageous.
Also known are carriers coated with a VdF/TFE/vinyl butyrate copolymer (Japanese Unexamined Patent Publication No.54-110,839). This type of carrier has a moderate and suitable electrostatic charge capacity. However, the coating layer formed of the copolymer is soft and easily damaged when stirred with the particulate toner and low in durability.